Subduction of seamounts and ridges along thinly sedimented convergent margins results in deformation of the overriding forearc. Exposures of newly recognized late Pleistocene, shallow water deposits (i.e., Marenco formation) record intervals of rapid subsidence and uplift across the Costa Rican forearc inboard of the subducting Cocos Ridge. In general, the Marenco formation is a fining upward, fossiliferious, late Pleistocene, marine sand disconformably overlying beveled surfaces cut across the competent Osa m¿lange basement. The ~50 to 27 ka age of the Marenco formation is constrained by 12 accelerator mass spectrometry and two conventional 14C dates obtained on marine macrofossils. The deposition of this sequence coincident with a general fall in sea level during oxygen isotope stage 3 requires >6 mm yr-1 subsidence inboard of the northwest flank of the subducting Cocos Ridge. Presently, exposures of the Marenco formation are found at >75 m above sea level, requiring uplift rates in excess of 6 mm yr-1. We interpret the down and up history of vertical tectonism recorded by the Marenco formation as the response of the upper plate to variations in the elevation of the subducting Cocos Ridge. On the basis of a model where the upper plate deforms through bends because of roughness on a rigid downgoing plate, the rate, duration, and spatial distribution of vertical tectonism across the forearc are determined by the magnitude of the orthogonal component of the relative convergence vector and the bathymetry of the underthrusting plate. Application of this model to bathymetric data for the Cocos plate offshore yields a broad agreement between predicted future rates of subsidence and uplift and rates over the last 50 kyr recorded by the Marenco formation. Furthermore, analysis suggests that the arrival of the blunt-tipped leading edge of the Cocos Ridge (0.5--3 Ma) resulted in an initial period of very rapid (~30 mm yr-1) uplift.